1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image in an image-forming process such as electrophotography and electrostatic printing, and to an image-forming method employing the toner. More specifically, the present invention relates to a toner for developing an electrostatic image suitable for fixation of a developed image by heating and pressing onto a transfer medium, and to an image-forming method employing the toner.
2. Related Background Art
Many electrophotographic processes are known, for example, as disclosed by U.S. Pat. No. 2,297,691, and Japanese Patent Publication Nos. 42-23910 and 43-24748. Generally, in the electrophotographic process, an electrostatic image is formed by an appropriate method on a photosensitive member containing a photoconductive material, the electrostatic image is developed with a toner, the toner image is transferred onto a transfer medium like a paper sheet, and the transferred toner image is fixed by a suitable method such as heating, pressing, hot-pressing, and solvent-vapor treatment to obtain a copy or a print. In the electrostatic process, the untransferred toner remaining on the photosensitive member is cleaned by an appropriate method, and the above steps are repeated.
In a typical full-color or multi-color image formation system, color image formation is conducted as follows. A photosensitive drum or a photosensitive belt is electrically uniformly charged. The charged drum or belt is exposed imagewise to laser light modulated for one color image signal, for example, magenta color image signal, to form an electrostatic image on the photosensitive drum or belt. The electrostatic image on the photosensitive drum or belt is developed with a magenta-developing device to form a magenta toner image. The magenta toner image is transferred onto an intermediate transfer member. In the same manner, development and transfer of an image of a second color, a third color, and a fourth color are conducted. The superposed four-color toner image is transferred from the intermediate transfer member to a transfer medium. The four-color toner image is fixed on the recording member by a hot-pressing fixing means to form a full-color or multi-color image.
In another full-color or multi-color image formation employing four photosensitive drum and a transfer belt, color image formation is conducted as follows. A first color toner image is formed on a first photosensitive drum, a second color toner image is formed on a second photosensitive drum, a third color toner image is formed on a third photosensitive drum, and a fourth color toner image is formed on a fourth photosensitive drum. With movement of a transfer medium on a transfer belt, the first to fourth color toner images on the first to fourth photosensitive drums are successively transferred onto the transfer medium. The transferred first to fourth color toner images are fixed on the transfer medium by a hot-pressing fixing means to form a full-color or multi-color image.
In a still another full-color or multi-color image formation employing one photosensitive drum (or a photosensitive drum) and a transfer drum to hold a transfer medium, color image formation is conducted as follows. A first color toner image is formed on a photosensitive drum. The first color toner image is transferred from the photosensitive drum onto a transfer medium held on a transfer drum. In the same manner, second to fourth toner images are successively transferred onto the transfer medium on the transfer drum. The transfer medium having the first to fourth color toner images is separated from the transfer drum, and is delivered to a hot-pressing fixing means to fix the images to form a full-color or multi-color image.
In the full-color or multi-color image formation, in which plural color toner images are superposed, and in which plural color toner images are superposed, and is fixed by heat and pressure, low-temperature offset or high-temperature offset is liable to be caused as compared with mono-color image formation.
For imparting high fixability and high offset-resistance to the toner itself, methods have been disclosed in which a low-molecular wax like polyolefin wax is incorporated into the particulate toner: for example, in Japanese Patent Publication Nos. 52-3304, 52-3305, and 57-52574; Japanese Patent Application Laid-Open Nos. 58-215659, 60-217366, 60-252361, 62-14166, 1-109359, 2-79860, and 3-50559.
A toner containing a montan type wax, a mineral wax, is disclosed as a wax component giving relatively high fixability. As the montan type wax, use of a wax of the formula below: ##STR1## (where R is a hydrocarbon group having 28 to 32 carbons, and n is an integer) having a molecular weight of about 800 is disclosed in Japanese Patent Application Laid-Open Nos. 1-185660 and 1-238672. However, this montan type wax, which has a straight molecular structure, has high plasticizing effect to soften the toner to deteriorate the high-temperature offset resistance, although low-temperature fixation can be improved in some extent. The plasticized toner is liable to cause disadvantages in development properties, durability, and anti-blocking property.
A toner containing partially esterified polyglycerin compound is disclosed in Japanese Patent Application Laid-Open No. 4-184350. The polyglycerin is, for example, represented by the formula below: ##STR2## (where n is an integer of one or more). The partially esterified polyglycerin is an ester of the polyglycerin having remaining free OH groups, not containing substantially completely esterified polyglycerin having no free OH groups. The partially esterified polyglycerin compound, although it has higher affinity to paper sheets owing to the remaining OH groups, is liable to lower the triboelectric charging properties and to lower the resolution. Further, in the case where toner particles are directly formed from a polymerizable monomer composition in an aqueous medium, the remaining OH groups of the partially esterified polyglycerin contained in the monomer composition will retard the particle formation in the aqueous medium, and broaden the particle size distribution of the formed toner particles.
Formation of a toner particles by suspension polymerization is disclosed in Japanese Patent Publication 36-10231. In this suspension method, a polymerizable monomer composition is prepared by dissolving or dispersing uniformly a polymerizable monomer, and a colorant (and, as necessary, a polymerization initiator, crosslinking agent, a charge-controlling agent, a wax, and other additives); the polymerizable monomer composition is dispersed as particles in an aqueous medium containing a dispersion stabilizer by means of an appropriate stirrer; and the dispersed monomer composition is allowed to polymerize to form a toner particles.
In the suspension polymerization, liquid drops of a polymerizable monomer composition are formed in a highly polar dispersion medium like water, so that the polar groups in the polymerized monomer composition tends to locate at the surface layer portion of the particle at the interface, and the nonpolar components tend to be in interior of the particle, which gives a core/shell structure.
The particulate toner formed directly by polymerization encloses the wax component as a releasing agent in the interior. Therefore, the toner can satisfy simultaneously low-temperature fixability, and anti-blocking property and high-temperature offset resistance which are inherently incompatible, and can prevent high-temperature offset without applying a releasing agent like an oil on the fixing roller. For this purpose, even though toner particles are prepared by polymerization method, the ester wax capable of forming favorable toner particles is promising.